Skykomish Railroad Lines

Jacobs Associates designed and provided construction management for a trenchless drive 90 feet (27.4 m) long and 36 inches (1 m) in diameter. This drive is part of the Hydraulic Control and Containment system (HCC) and runs beneath three active BNSF Railway lines in Skykomish, Washington. The HCC is part of a groundwater recovery system being installed to remediate the area, which historically had been a maintenance yard and refueling facility. A utility conduit was required to pass underneath the busy rail tracks to accommodate a gravity waterline, pressure waterline, compressed air line, power conduit, and signal wire.

Jacobs Associates, as a subconsultant to AECOM, performed a geotechnical investigation at the site of the proposed utility crossing and developed geotechnical design criteria to evaluate the feasibility of different trenchless construction methods. Several potential risks were identified during this process, including obstructions, shallow groundwater levels within 10 feet (3 m) of the surface, and the possibility of settlement and surface heave. The site geology is mostly glaciofluvial sediments—sand and gravel, with cobbles and boulders.

With 30 Class 1 freight trains running through the town per day and difficult ground conditions, we needed to find the best construction solution to minimize disruption to the railroad and meet budget and schedule constraints. After carefully examining several different trenchless methods, we chose pipe ramming, primarily for its ability to handle cobbles and boulders and to minimize the risk of settlement of the railroad tracks. In addition to its applicability to the site, pipe ramming provided a fast, efficient, and cost-effective installation method. Construction of this shallow utility crossing was completed in 2008. An additional 12-inch-diameter (0.3 m), 175-foot-long (53 m) casing was installed under the railroad tracks by pipe ramming in 2009.